1. FIELD OF THE INVENTION
The invention relates to an electronic temperature control unit for a mixing valve, especially for plumbing fixtures in which hot and cold water are mixed together.
2. DESCRIPTION OF PRIOR ART
Electronic control of mixing valves is disclosed, for example, in German Letters of Disclosure Nos. 2,836,698, 3,030,716, 3,146,501, 3,407,796, 3,425,445, 3,430,176 and U.S. Pat. No. 4,420,881. The temperature setting made manually with a conventional mixing valve is intended to be accomplished with rapidity and precision by electronic means, and to be maintained even where changes in supply pressure and temperatures occur.
As in the case of a manually actuated mixing valve, various mixing systems may be employed, for example, separate valves for mixing hot and cold water, or piston or disk type valves in which the controls for hot and cold water are combined in a single actuating unit.
The object of establishing and maintaining a certain outlet temperature is achieved by means of a regulator, which may comprise proportional, integral or differential (P, I, D) components or combinations thereof.
A disadvantage of known control systems is that the loop gain (control circuit amplification) used must be small, and hence the outlet temperature of the mixing valve cannot be corrected quickly enough to achieve stable operation of the control system in all operating conditions, with supply pressures and temperatures differing within wide limits. Unstable operating conditions arise, for example, when the valve is wide open but is sharply throttled by an outlet resistance such as a shower head or a clogged aerator, which results in simultaneous differences in supply pressures between hot and cold water.
An improvement in the proportional component of the regulation is obtainable by measuring pressure perturbations in order to compensate for them. With exact measurement and reproducible properties of the valves used, the valve settings can in principle be so determined that the outlet temperature is maintained, unaffected by the perturbations.
Direct measurement of the supply pressures requires complex and expensive sensors and is best avoided; however German Letter of Disclosure No. 3,407,796 shows how the pressure perturbations .DELTA.p.sub.H, .DELTA.p.sub.C can be calculated from the cold, hot and mix temperatures T.sub.C, T.sub.H, T.sub.M and the valve opening settings. It is an advantageous feature that the pressure measurement can be reduced to temperature measurement and displacement or angular measurement (i.e. valve opening). It is a disadvantage in this procedure that the pressure variations, in themselves rapid, must be sensed and evaluated with a delay due to the time lags of the temperature sensors and an additional time lag for calculating the pressure ratio. A further disadvantage is that calculated valve settings, owing to unavoidable errors of measurement and irregularities of the valves, especially after some time in service, will not result in the exact target temperatures, thus leaving a residual temperature error to be compensated by an additional I-control which, in turn, will function comparatively slowly for the reasons given above.